Move to a generated (in dist) ChangeLog

I still do not like not being able to correct the commit messages
and I still do not like how it leads us to be lazy when writing
the ChangeLog entries, but git still makes it hard to manage
branches when using a manually-written ChangeLog.
AppData file: Fix validation problems.

* glom.appdata.xml.in: Fix problems reported by appdata-validate:
  Change _p tags to p and add an extra paragraph in the
  description.